Automatic volume control system



Aug. 19, 1941. C, TRAVlS 2,253,450

AUTOMATIC VOLUME CONTROL SYSTEM Filed June 1'7, 1939 figa. 7

antan-a a0 rPz/r Patented Aug. 19, 1941 AUTOMATIC VOLUME CONTROL SYSTEMCharles Travis, Philadelphia, Pa., assignor, by

mesne assignments, to PhilcoRadio and Television Corporation,Philadelphia, Pa., a corporation of Delaware Application June 17, 1939,serial No. 279,739

(c1. 25o-2o) 11 Claims.

This invention relates to an improved detector and automatic volumecontrol system for useV in conjunction with modulated carrier wavesignal systems, such as are employed in radio-telephony and intelevision. More particularly, the invention relates to a method ofderiving an automatic volume control signal from a detector of theso-called degenerative plate rectification or infinite impedance type.By the presentinvention, means are provided whereby automatic volumecontrol may be obtained with a minimum .of additional equipment and byemploying circuits considerably more simple than those heretofore used.v

Accordingly one object of the invention is to provide an improved methodof deriving automatic volume control voltage from a detector of thedegenerative plate rectification type.

Another object of the invention is to provide a, modulated carrier wavesignal system in which asingle space discharge deviceA is employed bothas a degenerative plate rectification detector and as asource ofautomatic volume control voltage to control the gain of a signaltransfer means prior to the detector stage, and which serves to amplifythe signal supplied thereto.

Y Still another object of the invention is to `providev an improvedmethod of obtaining a relativelylarge variation in automatic volumecontrolvoltage from a degenerative plate rectification detector for arelatively small variation in the magnitude of the signal applied to theinput thereof.`

A further object of the invention is to provide animproved method ofobtaining a delayed automatic volume control voltage from a degenerativeplate rectification detector. .Y A still further object of the inventionis to provide an improvod method of obtainingy directly from a sp-acedischarge' device employedV as a degenerative plate rectificationdetector a control voltage whichV varies in therequired sense for use incontrolling the gain of, signal transfer means preceding the detectorandserving to amplify the signal supplied thereto. The invention may beclearly understood by reference to the accompanying drawing in which;Fig. 1 is a diagrammatic illustration of one .form of the invention; y

a load impedance in its cathode circuit. This impedance serves not onlyas a load impedance for the detected output but also as a biasing means,by reason of the presence of which the grid is prevented from goingpositive when the input signal thereto increases. Preferably, theimpedancemaycomprise a parallel combination consisting of a resistor,the magnitude of` which is relatively large by vcomparison with thereciprocal transconductance of the tube, and a condenser of suii'icientcapacitance adequately to by-pass the carrier frequency component of thesignal to be detected. Preferably, no load impedance is used in theanode circuit of the space discharge device, since this Would Cause theanode potential to vary with the plate current which would impair theoperation of the tube as a detector. The potential of any point on thecathode load impedance increases positively With respect to ground asthe input signal to the detector increases, and hence the voltage acrosssaid impedance is not suitable for use directly to control the gain of avacuum tube amplifier supplying signal to the detector input.

Various devices have been devised heretofore to obtain automatic volumeor gain control when a degenerative plate rectification detector isemployed, but such devices have been objectionably complex and haverequired an unduly large complement of additional equipment. On theother hand, as will appear presently, this invention provides a systemwhich is simple, effective and requires very little additionalequipment. "Moreover, 'the invention lends itself to the incorporationof certain desirable features of Vother automatic volume controlsystems, as will be vseen later. K Referring rst toFig. 1 of thedrawing, which shows a'speciiic embodiment of the invention, l is asource 'of'modulated carrier wave signal, for example the antenna coilof a radio or television receiver. VV2 represents the various apparatuswhich may be located between the signal Vsource and Athe detector"tube.' In an ordinary superheterodyne radio receiver, for example, thismight comprise radiofrequency amplifier stages, a lirstV detector, andintermediate frequency ampliiier stages, all familiar to those skilledin the art and which, for convenience, will hereinafter be referred toas signal transfer means.

According to the present invention, the triode which Would commonly beused in a degenerative plate rectification detector is replaced by apentode 3. In the cathode circuit, is a load comprising the resistor 4and the condenser 5 which are selected in accordance with the usualprinciples employed in designing degenerative plate rectificationdetectors. Anode circuit power is supplied from the source 6, and animpedance 1 is provided connecting the anode to ground. By this means,the automatic volume control voltage which is obtained from the plate ofthe tube 3 is always negative with respect to ground and such as toreduce the gain of the stages to which automatic volume control isapplied. Of course, it would also be possible to connect the highvoltage terminal of the impedance 1 to a source of negative potential,for example -3 volts, Whereby the control voltage would never becomemore positive than this value. The input signal from the signal transfermeans may be supplied between the control grid of the tube 3 and the lowpotential terminal of the resistor 4. The detected signal may then bederived across the resistor A, and the automatic volume control voltageobtained from the plate of tube 3 may be supplied back to the grids ofthe tubes in radio frequency or intermediate frequency amplifier stagesor both, the control voltage being subject to the smoothing action ofthe filter resistor 8 and by-pass condenser 9.

Considering noW the operation of the system as described, it may firstbenoted that although the detector output is from the load in the cathodecircuit, 4automatic volume control voltage is obtained from the anodecircuit by the insertion of asuitable load wherebyv the voltage of theplate decreases as the space current thereto increases, thus giving thedesired change in control voltage. In a triode, this would beundesirable ysince lthe reduction in `plate voltage would tend furtherto reduce the plate cuurrent, thus modifying the current in the cathodeload and affecting the behavior of the detector. By the method of theinvention, advantage is taken of the fact that in a space dischargedevice of the pentode type the total cathode current is substantiallyindependent of the plate potential; yet the major part of the cathodecurrent is derived from the plate provided the latter does not fall toofar beloW thepotential of the screen grid. Hence a load may be includedin the anode circuit, thus permitting automatic volume control voltageto be derived which varies in the proper sense without disturbing theoperation of the detector. The latter functions just as if a triode werebeing used. It Will appear that one of the so-called beam power tubesmight be used in place of the pentode; in fact a tetrode might even beused provided that its plate potential is not permitted to fall to tooloW a value. The requirements to be met in the selection of a tube forthe purpose will be apparent to those skilled in the art from thepreceding discussion.

As observed heretofore, the present invention A a large change inautomatic gain control voltage results. This is achieved by means .of adiode connected in the plate circuit of the gain control tube. Bysuitably adjusting the b-ias applied to the diode, the action of thesystem may be delayed by any desired amount. 'Ihe use of a diodeaccording to Applegarths teaching appears in the embodiment of Fig. 2 inwhich elements corresponding to those 0f Fig. 1 are designated by likereference numerals and need no further explanation. An additional sourceof positive voltage may be included in the anode circuit at i0. Thediode Il may have its `anode connected to the plate of the tube 3 andits cathode may be l maintained at a point of predetermined, preferablynegative, potential by means of the source I2. It will appear that noautomatic volume control voltage can develop until the plate currient ofthe tube 3 has increased suliciently to permit the plate to assume apotential corresponding substantially to that of the cathode of theVdiode when the latter is non-conducting. Hence the delay will dependupon the potential at which the cathode of the diode is maintained, themagnitude of the plate impedance 1, and the magnitude of the voltageprovided by the source I0.

Under certain circumstances, it may be preferable to use a seriesarrangement of the diode of Fig. 2. Such an arrangement is shown in Eig.3, Wherethe cathode of the diode I3 is shown connected to the plate ofthe pentode, the plate of the diode being connected through a resistorI4 to a source Iof predetermined preferably negative potential I2. Here,as before, no automatic v01- ume control voltage can be developed untilthe plate of the tube 3 assumes a potential suubstantially as negativeas that of the diode anode when the latter will begin to conduct and theautomatic volume control Voltage Will develop across the resistor I4.The particular practical advantage o-f this arrangement over the oneshown in Fig.V 2 is that it is impossible for a positive control voltageto be applied to the grids of the'controlled tubes. This might happen inthe system as shown in Fig. 2 due to the failure of the diode cathode toheat up soon enough when the apparatus is put into operation. This mightbe objectionable when the device is employed, for example, in a homeradio set but may be completely avoided by using the circuit of Fig. 3.

It Will of course be understood that the invention is subject tomodcation, as will appear to those skilled in the art upon reading theforegoing description. Therefore the invention is not to be regarded aslimited to the specific embodiments shown but only by the scope of theappended claims. A

Iclaim:V

l. In an electrical signal modifying system; a source of a -modulatedcarrier Wave signal; a space dischargedevice having a cathode, at leastone input electrode, at least one output electrode, and being soconstructed and operated that the current from said cathode issubstantially independent ofthe potential of said output electrode; animpedance associated with said cathode so as to form in conjunctiontherewith a degenerative plate rectification detector having an inputcircuit and an output circuit; variable gain signal transfer meansforsupplying signal from said source to the input of Isaid detector;` meansassociated with said output electrode and adapted to vary the potentialthereof in accordance with the magnitude of the electron current ovvingthereto; means adapted to derive a control voltage substantiallydependent upon said output electrode potential whenever said potentialfalls below a predetermined value, said means comprising a diode spacedischarge device having its anode connected to said output electrode andhaving its cathode connected to a point of predetermined potential; andmeans for supplying said derived signal to said signal transfer means tocontrol the gain thereof.

2. In an electrical signal modifying system; a source of -a modulatedcarrier wave signal; a space discharge `device having a cathode, atleast one input electrode, at least one output electrode, `and being soconstructed and oper-ated that the current from said cathode issubstantially independent of the potential of said output electrode; animpedance associated with said cathode so as to form in conjunctiontherewith ya degenerative plate rectification detector having an inputcircuit and an output circuit; variable gain signal transfer means forsupplying signal from said source to the input of said detector; meansassociated with said output electrode and adapted to vary the potentialthereof in accordance with the magnitude of the electron current flowingthereto; means adapted to derive a control voltage substantiallydependent upon said output electrode potential whenever said potentialfalls below a predetermined value, said means comprising :a 4diode spacedischarge device having its cathode connected to said output electrodeand having its anode connected through an impedance to a point ofpredetermined potential; vand means for supplying said derived signal tosaid signal transfer means to control the gain thereof.

3. In an electrical signal modifying system.; a source of a modulatedcarrier wave signal; a pentode space discharge device having an anode, acathode, and control, screen'and suppressor grids, and being so operatedthat cathode current is substantially independent of variations in plateVoltage; an impedance associated with the Icathode of said spacedischarge device so as to form in conjunction therewith Ia, degenerativeplate rectification detector having an input circuit and an outputcircuit; variable gain signal transfer means for supplying signal fromsaid source to the input of said detector; a source of anode supplyvoltage; a resistance connecting the said supply to the said anode;means for bypassing said resistance for currents of carrier frequency; aconnection to the said anode for deriving a control voltage; and meansfor utilizing said control voltage to control the gain of said signaltransfer means.

4. In an electrical signalling system; a source of an electrical signal;a space discharge device having at least a cathode, an anode, and acontrol grid, means for rendering the space current in said devicesubstantially independent of th-e potential of said anode; variable gainsignal transfer means for supplying signal from said source to saidcontrol grid; a resistor 'connecting said anode to a point of fixedpotential; an impedance connecting said cathode to a point of relativelylower potential; means for deriving an output signal -across saidlast-named impedance; and a connection between said anode and saidsignal transfer means whereby the potential of said anode is applied tocontrol the gain of said signal transfer means.

5. In an electrical signalling system; a source of an electrical signal;a space discharge device having at least an anode, a cathode, and alco-ntrol grid; variable gain signal transfer means for supplying signalfrom said source to said control grid; a resistor connecting said anodeto apoint of predetermined-potential; an impedance connecting saidcathode to a point of relatively lower potential; means for deriving anoutput signal from said last-named impedance; and a connection betweensaid anode and said signal transfer means whereby the potential of saidanode is applied to control the gain of said signal transfer means.

6. In an electrical signalling system; a source of a modulated carrierwave signal; a space discharge device having at least a cathode, an

anode and a control grid; variable gain sig-nal transfer means forsupplying signal from said sourceto said control grid; an impedanceconnected to said cathode for forming in cooperation with the saidelements of said space'discharge device a degenerative platerectification -detector; means for deriving an output signal from saidimpedance; a second impedance connected to the anode of said spacedischarge device for developing a control voltage from the recticationvaction thereof; and means for utilizing said voltage to control the gainof said signal transfer means.

'7. In an electrical signalling system; a source of an electricalsignal; a space discharge device having at least an anode, a cathode,and a control grid; variable gain signal transfer means connecting saidsource to the control grid of said space discharge device; an impedanceconnecting said anode to a point of predetermined potential; a secondimpedance connecting said cathode to point of predetermined relativelylower potential to form a degenerative rectifying circuit including theanode-cathode path of said space discharge device; means for renderingthe current in said second impedance substantially independent ofvariations in the potential of said anode; means for deriving a signalproportional to the rectified current in said circuit; means forderiving a gain controlling voltage from said first impedance, and meansfor utilizing said last-named voltage to control the gain of said signaltransfer means.

8. In an electrical signalling system; a source of an electrical signal;a space discharge device having at least an anode, a cathode, and acontrol grid; variable gain signal transfer means connecting said sourceto the control grid of said space discharge device; an impedanceconnecting said anode to a point of predetermined potential; a secondimpedance connecting said cathode to point of predetermined relativelylower potential to form a degenerative rectifying circuit including theanode-cathode path of said space discharge device; means comprisingauxiliary electrodes in said space discharge device and supplied withsuitable potentials for rendering the current in said second impedancesubstantially independent of variations in the potential of said anode;means for deriving a signal proportional to the rectified current insaid circuit; means for deriving a gain controlling voltage from saidrst impedance, and means for utilizing said last-named voltage tocontrol the gain of said signal transfer means.

9. In an electrical signalling system; a source of an electrical signal;a space discharge device having at least an anode, a cathode, and acontrol grid; variable gain signal transfer means connecting said sourceto the control grid of said space discharge device; an impedanceconnecting said anode to a point of predetermined potential; a secondimpedance connecting said cathode to point of predetermined relativelylower potential to form a degenerative rectifying circuit including theanode-cathode path of said space discharge device; means for renderingthe current in said second impedance substantially independent ofvariations in the potential of said anode; means for deriving an outputsignal across said second impedance; means for deriving a gaincontrolling voltage from said first impedance, and means for utilizingsaid last-named voltage to control the gain of said signal transfermeans.

10. In an electrical signalling system; a source of a modulated carrierWave signal; a space discharge device including triode elementsconstituting a plate rectication detector; variable gain signal transfermeans connecting said source to said detector; means for rendering saiddetector degenerative; means for rendering the rectified current of saiddetector substantially independent of variations in the potential of itsanode;

means for deriving a detected signal from the rectied current of saiddetector; means for deriving a gain controlling voltage from the anodeof said detector; and means for applying said voltage to said signaltransfer means to control the gain thereof.

11. In an electrical signalling system; a source of a modulated carrierWave signal; a space discharge device including triode elementsconstituting a plate rectification detector; variable gain signaltransfer means connecting said source to said detector; means forrendering said detector degenerative; means comprising at least oneadditional element in said space discharge device for rendering therectified current of said detector substantially independent ofvariations in the potential of its anode; means for deriving a detectedsignal from the rectified current of said detector; means for deriving again controlling voltage from the anode of said detector; and means forapplying said voltage to said signal transfer means to control the gainthereof.

CHARLES TRAVIS.

